The present invention relates to a postage meter (also known as a franking machine) having a franking head fixed to one end of a machine main shaft and in which are adjustably mounted franking value or stamp index wheels. The wheels are in driving connection with a meshing tooth system of racks which are guided in the machine shaft and which additionally have a driving tooth system and a position sensing tooth system. At least one electromagnet engages in each case one adjusting ram with the driving tooth system of a rack and on the driving tooth system of each rack is provided a pawl preventing the rearward movement of the rack. On the position sensing tooth system of the rack there is at least one position sensing pawl additionally securing the adjustment position.
A postage meter or franking machine of this type is known from U.S. Pat No. 4,520,725. A given franking value is set by electromagnetically driven adjusting rams or plungers which move a rack forwards in the direction of the franking head by a number of reciprocating movements corresponding to the numerical value to be set, so that the particular franking value or stamp index wheel of the franking head is set to this numerical value. A pawl is provided for each rack and following each displacement movement of the rack engages behind one of the teeth of its driving tooth system and consequently secures it in the set position where the values of the franking value printing wheels may be permissibly changed by the tension of a return spring. Sensors positioned on the pawls indicate any movement of a pawl over and beyond a tooth of the driving tooth system and into the next tooth space to the electronic display register. Position sensing pawls are also provided which, during franking, engage in an additional position sensing tooth system of the rack and consequently fix the set position of the franking value printing wheels.
In the construction described in the aforementioned U.S. patent, the inhibiting of the displacement movement of a rack due to unauthorized use or manufacturing inaccuracies can lead to the rack return spring not moving back into its initial or neutral position when a new franking value is to be set, so that the new set value is added to a residual value, which does not enter the register for the amounts consumed. If the rack sticks in an intermediate position, then the associated pawl remains on the tip of a tooth of the driving tooth system and the sensor indicates the zero value of the amount count to the register. If the franking machine drive is switched on for franking purposes in such an intermediate position of the rack, damage also occurs to the retaining pawl mechanism, because its cam drive cannot move the retaining pawls into a tooth space of the retaining tooth system.